A conventional aluminum and fiberglass superinsulated Dewar uses several principles to provide insulation for the purpose of holding a cryogen (i.e., a cryogenic volatile liquid, usually helium) in a vessel with minimal losses. As used herein, "volatile" means a liquid which boils readily, i.e. it has a low latent heat and a normal liquid temperature below room temperature.
In a typical, conventional Dewar, the inner helium reservoir (belly, tail, and the lower part of the neck) is surrounded by vacuum. In the vacuum, which surrounds the inner helium reservoir, heat is not conducted by convection, lattice conduction, electronic conduction or other mechanisms as would occur in gases, liquids or solids. However, in the vacuum there is heat conducted by thermal (blackbody) radiation. To reduce this channel of heat transfer to the volatile cryogenic liquid, thin and reflective material is placed between the warm and cold surfaces to intercept and reflect the radiation. Even non-reflective material has an insulating effect, but reflectivity improves the insulating effect, and the more reflective (lower the emissivity), the better the insulating effect. The insulating effect also increases with the number of layers of material between the warm and cold surfaces. However, the greater the number of layers, the greater is the chance that heat will be conducted through the insulation by conventional means (via the thermal conductance through the solid which comprises the layers). Therefore, the insulating layers in the vacuum chamber of a typical Dewar are comprised of thin, low conductivity materials (aluminized mylar, typically, where the aluminum layer is very thin). The aluminized mylar layers may be separated by isolating layers of material, such as bridal veil or similarly gossamer, thin, insulating material. With any superinsulating layer design incorporating the above materials and techniques, there is an optimal packing density; techniques for maintaining this density involve slow and careful wrapping so that the correct density is achieved.
Once radiation loads of the reservoir have been substantially reduced, there remains the thermal load conducted down the neck. The neck in a typical Dewar is comprised of a material with good mechanical strength, such as a thermally insulating glass reinforced epoxy (fiberglass) or thin stainless steel. The diameter and the wall thickness of the neck tube are reduced to the minimum given other constraints of the Dewar; and the neck is made as long as mechanical, geometric, and experimental constraints allow. Some of the constraints, such as neck diameter, are determined by the use to which a Dewar is applied. Other constraints, such as the height of the Dewar--and therefore the neck length--may be determined by ceiling height. The use of the Dewar for a specific purpose of cooling an apparatus, such as a magnet or cryostat may also affect these parameters.
Even when the thermal looses from the neck and from radiation are reduced to the greatest extent possible, the conventional Dewar also uses escaping gas (which inevitably is lost due to residual loads) to intercept heat. With helium this is especially important, since the latent heat (the amount of heat necessary to boil a given quantity of liquid) is very low. The heat necessary to raise the temperature of the gas back to ambient (room) temperature (295 K., or 22.degree. C. or 70.degree. F.) is much greater (about 60 times) than the latent heat. Thus, the Dewar makes use of the change in enthalpy (change in heat content) of the gas by forcing gas flowing in the neck to carry away heat which would otherwise be conducted or radiated to the liquid helium in the belly and tail. The helium is directed to rise along the neck to make contact with the neck and to remove heat from the neck. Baffles thus may be included in the neck to force the flow of helium in this manner.
Another insulating technique provides cooled shields to dynamically intercept the radiation load through the superinsulation, just as the helium escaping dynamically intercepts the neck load. Thermal anchor shields may be provided at strategic locations along the neck where the escaping gas makes contact. The shields then intercept heat flow through the superinsulation and carry it to the escaping gas.
Finally, another insulating technique for a Dewar is to use a less expensive and less volatile cryogen (typically liquid nitrogen) to intercept heat before that heat reaches the more expensive, more volatile, and lower temperature cryogen (typically liquid helium). A reservoir or flow of liquid nitrogen is positioned where it can intercept heat traveling down the neck tube, and a liquid-nitrogen-cooled shield is placed in the superinsulation completely surrounding the helium reservoir to intercept residual heat flow through the superinsulation.